1. Field of the Invention
This invention relates to a vertical-cavity surface-emitting laser device, and in particular relates to a vertical-cavity surface-emitting laser device which is substantially independent of a temperature and can emit at oscillation wavelength as we like. This invention also relates to a process for fabricating the vertical-cavity surface-emitting laser device and a lasing process. In addition, this invention relates to a vertical-cavity surface-emitting laser device capable of multi-wavelength lasing and a multi-wavelength lasing process. Furthermore, this invention relates to an optical communication system employing the vertical-cavity surface-emitting laser device.
2. Description of the Related Art
In the field of data communication, a data rate has been rapidly increased, and data throughput and bandwidth have been enlarged. Thus, much attention has been attracted to optical communication, and this technique has been employed. Using light may eliminate influence of electromagnetic-wave noises such as EMI from a cable, leading to large-capacity transmission with a light-weight cable. Recently, some optical-module products have been available for Fiber Channel (1.0625 Gbps) and Gigabit Ethernet (1.25 Gbps).
For high-speed and large-capacity transmission, a wavelength division multiplex (WDM) technique has been developed. A vertical-cavity surface-emitting laser (VCSEL) has been developed as a low-cost light source for high-speed data link and incorporated in the above optical modules. Features of VCSELs are as follows.
1) VCSELs are low-cost devices, because they allow some on-wafer tests and a cleavage for forming mirror edges is not required.
2) VCSELs have a small light beam divergence, which allows a high coupling efficiency with optical fibers.
3) VCSELs have a higher reliability than CD lasers.
One of the properties required as a light source is a good temperature characteristic. Specifically, if its performance significantly depends on a temperature (typically, a higher temperature deteriorates its performance), power consumption may be increased, which may accelerate deterioration, even when an automated power control (APC) is employed for a constant light output.
In edge-emitting lasers, such characteristic deterioration with an increase in temperature can rarely be prevented, and attempts have been made to reduce such deterioration.
To obtain good temperature characteristics, in the prior art for VCSELs, a gain peak is set to a shorter wavelength side than a cavity-resonant peak (gain offset) to allow these two peaks to overlap as temperature rises and thus to prevent characteristic deterioration (the gain peak and the cavity-resonant peak are shifted to a longer wavelength direction at a rate of about 0.3 nm/xc2x0 C. and about 0.07 nm/xc2x0 C., respectively). D. B. Young et al. have detailed such a technique in IEEE Journal of Quantum Electronics, Vol.29, 2013-2022 (1993). Alternatively, there is an attempt made that the wavelength of gain peaks for each of quantum wells in active layers differs from each other to increase a net gain bandwidth for providing a constant gain in a wide temperature range. The technique has been described in M. Kajita et al., IEEE Journal of Selected Topics in Quantum Electronics, Vol.1, pp.654-660 (1995) or JP-A 7-245449.
According to the above gain offset method, within the range shown in FIG. 8, a threshold current of a device can remain low for VCSELs. Note that a threshold current may increase as a temperature rises, for edge-emitting lasers.
These techniques, however, require precise control during wafer growth, and thus a yield is not always satisfactory.
Thus, in JP-A9-8413, the present inventor has suggested a process for manufacturing a VCSEL where active layers are grown at a low temperature to avoid deviation of a gain offset from a designed value due to, for example, film-thickness distribution and flux drift inevitable during the growing step, and after growing, data on cavity-resonant and gain peaks are obtained, based on which the device is heated at a selected temperature to achieve a desired offset between the two peaks and thus to consistently ensure excellent designed characteristics.
As described above, the VCSEL thus obtained may have constant characteristics in some wide temperature range. However, since the designed range cannot be altered, characteristic deterioration is inevitable beyond the range. Thus, in the prior art, a light intensity can be controlled, while a wavelength is controlled only by adjustment during a manufacturing process, but not after the process.
An objective of this invention is to provide a VCSEL substantially independent of a temperature.
Another objective of this invention is to provide a VCSEL whose wavelength can be controlled after manufacturing thereof.
Another objective of this invention is to provide a lasing process, in particular a multiwavelength lasing process, using the VCSEL.
The above objectives can be achieved by a VCSEL of this invention comprising
a surface light-emitting part comprising a resonator consisting of two multilayer reflectors and active layers inserted between two reflectors; and
a waveguide part for introducing induced light near the surface light-emitting part.
The VCSEL can be produced by a process comprising sequentially depositing at least an n-type multilayer reflector, an n-type intermediate layer, an active layer, a p-type intermediate layer and a p-type multilayer reflector on a substrate;
etching the p-type multilayer reflector to form a mesa structure for a surface light-emitting part, and etching a predetermined region to form a mesa structure for a waveguide part;
forming p-type electrodes on the mesa structures of the surface light-emitting part and of the waveguide part;
forming a high-resistive region; and
forming an n-type electrode on the back of the substrate.
For a lasing process using the VCSEL, the cavity-resonant peak is preset, at a given temperature, to a longer wavelength than the gain peak, and an applied voltage is adjusted to make the gain peak be aligned with the cavity-resonant peak when induced light generated by applying the voltage to the waveguide part is introduced to the surface light-emitting part.
Multiwavelength oscillation using the VCSEL can be achieved by presetting the cavity-resonant peak, at a given temperature, to a longer wavelength than the gain peak, adjusting a voltage applied to the waveguide part so that the gain peak be aligned with the cavity-resonant peak by induced light generated at the waveguide part into the surface light-emitting part, for laser oscillation at one wavelength; then altering the device temperature to shift the cavity-resonant peak; and again adjusting the applied voltage to shift the gain peak to be aligned with the cavity-resonant peak, for laser oscillation at a different wavelength from the previous wavelength.
Furthermore, this invention provides an optical communication system comprising at least a transmission part, a corresponding receiving part and a light transmission means connecting between them, characterized in that the above VCSEL is used as a light source for the transmission part to allow the single light source to emit one or more wavelength lights, and the light transmission means connects the single transmission part with one or more receiving parts.
According to this invention, a light source for an optical data link substantially independent of a temperature can be manufactured, which eliminates corrective actions such as temperature control even under conditions involving temperature rising inside of an apparatus such as computers. Specifically, temperature dependent factors may include a light output intensity and a wavelength. The light output intensity has been well controlled by the prior art, while a wavelength cannot be adequately controlled. There may be no needs for wavelength control in current replacement of a module with a single-wavelength. Wavelength control is, however, essential for coping with a future multiwavelength communication such as WDM. Such wavelength control can be performed after manufacturing the VCSEL, to improve a yield. Wavelength control can be precisely made during operation, which permits the VCSEL to be used for WDM applications.
A waveguide manufactured according to this invention does not alter a general process for manufacturing a VCSEL, and does not adversely affect a yield or assembly of the VCSEL.